Yesterday T-Mobile posted scant details regarding a mysterious Android system update for the Nexus 4 with the build number JWR66Y. Then the Galaxy Nexus started seeing the update. Now it looks like the update might have hit at least one Nexus 10. Unfortunately, we still don't know what the heck it does.
T-Mobile only listed "Security" under the improvements header. So, presumably it's patching some manner of system exploit that was discovered.